A variable area flow meter as shown in Hedland U.S. Pat. No. 3,805,611 typically has a cylindrical body with an inlet at one end and an outlet at the other. A piston inside the body is spring biased toward the inlet and is moved toward the outlet by fluid flow through the body. A conical plug extends through a hole in the end of the piston and is tapered toward the outlet. As the piston is moved against the spring by increased fluid flow, the opening for flow in the end of the piston grows larger. The piston reaches a stable position in the cylindrical body which is a measure of flow. If the cylindrical body is transparent, a scale on the body indicates the flow.
Where the cylindrical body is not transparent, the Hedland '611 patent incorporates a magnet in the piston with a follower of magnetic material outside the cylindrical body. The follower and a scale indicate the flow. Electrical output signals have been provided by switches with contacts actuated by the follower at selected flow rates. A remote electrical read out is shown in Lake U.S. Pat. No. 4,487,077. A resistor mounted on the cylindrical body is contacted by a wiper carried on the magnet follower and connected with a remote read-out. The meter of the '077 patent is subject to decoupling of the follower from the magnet as a result of rapid change in flow rate and exacerbated by friction between the wiper and the resistor.